1. Global climate change and shifting land-use are increasing plant stress due to abiotic factors such as drought, heat, salinity and cold, as well as via the intensification of biotic stressors such as herbivores and pathogens. The ability of plants to tolerate such stresses is modulated by the bacteria and fungi that live on or inside of plant tissues and comprise the plant microbiome. However, the impacts of diverse classes of beneficial microbes and the contrasting stresses that impact plant performance are most commonly studied independently of each other. 2. Our meta-analysis of 288 experiments across 89 studies moves beyond previous studies in that we simultaneously compare the roles of bacterial versus fungal microbiome members that live within plant tissues and colonize plant surfaces in ameliorating biotic versus abiotic sources of plant stress. 3. The magnitude of microbial stress amelioration can be measured as the greater proportional impact of beneficial microbes on plant performance in more stressful environments. In the plant experiments we examine, the magnitude of microbial stress amelioration is substantial: it is 23% of the effect size of the typical impact of stress and 56% of the effect size of beneficial microbes in the absence of stress. 4. The amount of benefit microbes confer to plants differs among classes of microbes, depending on whether plants are grown in stressful or non-stressful environments. In the absence of stress, beneficial bacteria tend to confer greater plant benefits than do fungi. However, symbiotic fungi, especially arbuscular mycorrhizal fungi, more strongly ameliorate plant stress than do bacteria. In particular, beneficial microbes ameliorate salinity, foliar herbivory, and fungal pathogen stress. 5. These results highlight the fact that the impacts of beneficial and antagonistic components of the microbiome on plant performance depend on biotic and abiotic environmental contexts. Furthermore, beneficial microbiota are especially critical for plant health in stressful environments and thus present opportunities to mitigate negative consequences of global change.